docs/pwg/advanced_types.xml: Finish documenting the current state of mimetypes.

docs/pwg/advanced_types.xml: Finish documenting the current state of mimetypes.

Original commit message from CVS:
2004-01-27  Ronald Bultje  <rbultje@ronald.bitfreak.net>

* docs/pwg/advanced_types.xml:
Finish documenting the current state of mimetypes.
* docs/pwg/building_boiler.xml:
* docs/pwg/building_chainfn.xml:
* docs/pwg/building_pads.xml:
* docs/pwg/building_props.xml:
* docs/pwg/building_testapp.xml:
Start documenting the "how to build a simple audio filter" part
of the PWG. Most stuff is ready by now. Stuff remaining: signals,
states and (maybe?) a short introduction to capsnego in the chapter
on pads (building_pads.xml). Capsnego should probably be explained
fully in advanced_capsnego.xml or so.